1. Field of the Invention
The present invention may relate to a field-effect transistor, a display device having the same, and a method of manufacturing the same. More particularly it may relate to a vertical field-effect transistor, a display device having the same, and a method of manufacturing the same.
2. Description of the Related Art
Organic electronic engineering may be applied to a display device, such as an organic light emitting diode, and to a field-effect transistor. Organic electronic engineering also may realize a single electronic circuit that can be manufactured without a complicated semiconductor manufacturing process. The circuits manufactured by a method based on an organic polymer semiconductor may be used to activate large size displays and also may be used in the field of transponders.
In a field-effect transistor structure based on organic materials, a semiconductor organic material may be arranged between a source electrode and a drain electrode. An electric field may be generated in an area (for example, a channel) between the source electrode and the drain electrode by applying a predetermined voltage to a gate electrode. As a result, charge carriers, (for example, electrons or holes) may be introduced into the channel formed of the organic material and may increase conductivity between a source contact and a drain contact. In this case, a line in a channel of a p-type transistor structure may be realized through the holes, and a line in a channel of an n-type transistor structure may be realized through the electrons. The transistor may be controlled by a gate voltage.
A charge carrier block layer, (for example, a dielectric layer) may be arranged between the channel and the gate electrode, and may prevent the migration of corresponding charges from the gate electrode to the channel. Such migration may deteriorate the quality of a desired electric field and of the transistor structure.
Such a structure may have a low maximum attainable current. The maximum attainable current may be an important factor of the adaptability of an organic transistor, such as one for use in an active matrix OLED display.
The maximum attainable current is determined based on the width and the length of a conductive channel (which is formed of an organic semiconductor material) and the charge carrier movement. Examples of organic materials include small molecular compounds like perylene tetra carboxylic acid diimide as an n-type semiconductor and diimide derivative of naphthaline tetra carboxylic acid diimide, as well as pentacenes, tatracences, and oligo thiophenes as p-type semiconductors. Other examples of organic materials may include polymers like a copolymer of alkylfluorene unites of alkythiophenes and polyalkylthiophenes.
Having the length of the channel small and the width and the movement of the channel large may help to obtain a maximum current. Another factor for obtaining the maximum current is the thickness of the organic semiconductor that defines the thickness of the channel. A channel used for the current modulation between the source electrode and the drain electrode may operate on a very thin layer that is located near the charge carrier block layer. Accordingly, it may not be necessary to increase the thickness of the organic semiconductor. Thicker semiconductor layer may deteriorate the current ratio in a switch-on state and a switch-off state. The optimum thickness of the semiconductor layer may be less than about 100 nanometers.
The charge carrier movement of the organic semiconductor may be significantly less than the charge carrier movement of an inorganic material, such as silicon. The typical movement of the organic semiconductor may be in a range from about 10−2 to about 1 cm2/Vs. Accordingly, it may be necessary to balance out the small charge carrier movement by reducing the length of the channel in order to obtain a large maximum attainable current of an organic semiconductor that has small charge carrier movement. The typical channel length of an organic field-effect transistor is about 5 to about 100 micrometers. In general, a high resolution process, such as photolithography, may be performed to lower the channel length to less than about 5 micrometers. Such a method may have a higher cost and may destroy the value of the organic electronic engineering.
In order to avoid a photolithography process requiring a high cost and in order to obtain a parallel structure with a high resolution and reduce the channel length in order to obtain a higher current various solutions have been attempted. To this end, a method of depositing a source electrode and a drain electrode of an organic field-effect transistor while the source electrode and the drain electrode are not across each other and adjacent to each other is discussed in “Thin Solid Films, Vol 331(1998), pp. 51-54” by Kudo et al. and “Science, Vol 299(2003), pp. 1881-1884” by Stutzmann et al., which are incorporated herein by reference in their entirety.
In Kudo, the source electrode and the drain electrode are arranged on the substrate to overlap each other but not to be adjacent to each other. When the gate electrode is continuously arranged to the organic material, the flow of charge carriers from the source electrode to the drain electrode may be disrupted. More specifically, a large leakage current from the gate electrode to the source electrode may destroy the value of a field-effect transistor that can be controlled without using an electric power.
In Stutzmann a small portion of the surface of the transistor is used as the channel, but a large portion of the surface of the transistor may be needed for use as the channel, in order to obtain a large current by using a small sized transistor.